Free-standing metallic article for semiconductors

ABSTRACT

A free-standing metallic article, and method of making, is disclosed in which the metallic article is electroformed on an electrically conductive mandrel. The mandrel has an outer surface with a preformed pattern, wherein at least a portion of the metallic article is formed in the preformed pattern. The metallic article is separated from the electrically conductive mandrel, which forms a free-standing metallic article that may be coupled with the surface of a semiconductor material for a photovoltaic cell.

RELATED APPLICATIONS

This application is related to 1) Babayan et al., U.S. patentapplication Ser. No. 13/798,124, entitled “Free-Standing MetallicArticle for Semiconductors” and filed on Mar. 13, 2013; 2) Babayan etal., U.S. Provisional Patent Application No. 61/778,443, entitled“Free-Standing Metallic Article for Semiconductors” and filed on Mar.13, 2013; and 3) Babayan et al., U.S. Provisional Patent Application No.61/778,444, entitled “Free-Standing Metallic Article for Semiconductors”and filed on Mar. 13, 2013; all of which are owned by the assignee ofthe present application, and are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

A solar cell is a device that converts photons into electrical energy.The electrical energy produced by the cell is collected throughelectrical contacts coupled to the semiconductor material, and is routedthrough interconnections with other photovoltaic cells in a module. The“standard cell” model of a solar cell has a semiconductor material, usedto absorb the incoming solar energy and convert it to electrical energy,placed below an anti-reflective coating (ARC) layer, and above a metalbacksheet. Electrical contact is typically made to the semiconductorsurface with fire-through paste, which is metal paste that is heatedsuch that the paste diffuses through the ARC layer and contacts thesurface of the cell. The paste is generally patterned into a set offingers and bus bars which will then be soldered with ribbon to othercells to create a module. Another type of solar cell has a semiconductormaterial sandwiched between transparent conductive oxide layers (TCO's),which are then coated with a final layer of conductive paste that isalso configured in a finger/bus bar pattern.

In both these types of cells, the metal paste, which is typicallysilver, works to enable current flow in the horizontal direction(parallel to the cell surface), allowing connections between the solarcells to be made towards the creation of a module. Solar cellmetallization is most commonly done by screen printing a silver pasteonto the cell, curing the paste, and then soldering ribbon across thescreen printed bus bars. However, silver is expensive relative to othercomponents of a solar cell, and can contribute a high percentage of theoverall cost.

To reduce silver cost, alternate methods for metallizing solar cells areknown in the art. For example, attempts have been made to replace silverwith copper, by plating copper directly onto the solar cell. However, adrawback of copper plating is contamination of the cell with copper,which impacts reliability. Plating throughput and yield can also beissues when directly plating onto the cell due to the many stepsrequired for plating, such as depositing seed layers, applying masks,and etching or laser scribing away plated areas to form the desiredpatterns. Other methods for forming electrical conduits on solar cellsinclude utilizing arrangements of parallel wires or polymeric sheetsencasing electrically conductive wires, and laying them onto a cell.However, the use of wire grids presents issues such as undesirablemanufacturing costs and high series resistance.

SUMMARY OF THE INVENTION

A free-standing metallic article, and method of making, is disclosed inwhich a metallic article is electroformed on an electrically conductivemandrel. The mandrel has an outer surface with a preformed pattern,wherein at least a portion of the metallic article is formed in thepreformed pattern. The metallic article is separated from theelectrically conductive mandrel, which forms a free-standing metallicarticle that may be coupled with the surface of a semiconductor materialfor a photovoltaic cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Each of the aspects and embodiments of the invention described hereincan be used alone or in combination with one another. The aspects andembodiments will now be described with reference to the attacheddrawings.

FIG. 1A is a perspective view of a conventional solar cell.

FIG. 1B is a cross-sectional view of a conventional back-contact solarcell.

FIG. 2 shows a perspective view of an exemplary electroforming mandrelin one embodiment.

FIGS. 3A-3C depict cross-sectional views of exemplary stages inproducing a free-standing electroformed metallic article.

FIG. 4 provides a cross-sectional view of one embodiment of anelectrically conductive mandrel.

FIG. 5 provides a cross-sectional view of another embodiment of anelectrically conductive mandrel.

FIGS. 6A-6B are top views of two embodiments of metallic articles.

FIG. 6C is a cross-sectional view of section B-B of FIG. 6B.

FIGS. 6D-6E are partial cross-sectional views of yet further embodimentsof the cross-section of FIG. 6B.

FIGS. 6F-6G are top views of yet further embodiments of metallicarticles.

FIG. 7 is an exemplary flow chart of a process for manufacturing anelectroformed article and forming a semiconductor device such as a solarcell.

FIGS. 8A-8B are perspective views of exemplary solar cells fabricatedwith a free-standing metallic article.

FIG. 8C is a cross-sectional view of another embodiment of a solar cell.

FIG. 8D is a top view of exemplary metallic articles used in the solarcell of FIG. 8C.

FIGS. 9A-9B illustrate an embodiment of tailoring features of anelectroformed element.

FIGS. 10A-10B illustrate another embodiment of tailoring features of anelectroformed element.

FIGS. 11A-11C are cross-sectional views of stages of forming a metallicarticle with a dielectric transfer layer, in one embodiment.

FIG. 12 depicts a cross-sectional view of an embodiment of a metallicarticle being removed using a polymer sheet.

FIG. 13A-13B are cross-sectional views of an exemplary polymer layerbeing fabricated into a back-contact solar cell.

FIG. 14 shows an exemplary flow chart of a method of manufacturing apolymer layer with an electroformed article and forming a semiconductordevice such as a solar cell.

FIGS. 15A-15D provide perspective views of exemplary stages in using ametallic article as a mask for pattering a conductive layer on asemiconductor material.

FIG. 16 is an exemplary flow chart of a method of using a metallicarticle as a mask for pattering a conductive layer on a semiconductormaterial.

FIG. 17 is a cross-sectional view of an exemplary cylindrical mandrel.

FIG. 18 shows a cross-sectional view of an embodiment of a flat mandrelhaving patterns on its top and bottom surfaces.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1A is a simplified schematic of a conventional solar cell 100 whichincludes an anti-reflective coating (ARC) layer 110, an emitter 120, abase 130, front contacts 140, and a rear contact layer 150. Emitter 120and base 130 are semiconductor materials that are doped as p+ or n−regions, and may be referred to together as an active region of a solarcell. Front contacts 140 are typically fired through anti-reflectivecoating layer 110 in order to make electrical contact with the activeregion. Incident light enters the solar cell 100 through ARC layer 110,which causes a photocurrent to be created at the junction of the emitter120 and base 130. It can be seen that shading caused by front contacts140 will affect the efficiency of the cell 100. The produced electricalcurrent is collected through an electrical circuit connected to frontcontacts 140 and rear contact 150. A bus bar 145 may connect the frontcontacts 140, which are shown here as finger elements. Bus bar 145collects the current from front contacts 140, and also may be used toprovide interconnection between other solar cells. The assembly of frontcontacts 140 and bus bar 145 may also be referred to as a metallizationlayer. In other types of solar cells, a transparent conductive oxide(TCO) layer may be used instead of a dielectric-type ARC layer, tocollect electrical current. In a TCO type of cell, metallization in theform of, for example, front contacts 140 and bus bar 145 would befabricated onto the TCO layer, without the need for firing through, tocollect current from the TCO solar cell.

FIG. 1B illustrates a simplified schematic of another type of solar cell160, in which the electrical contacts are made on the back side,opposite of where light enters. Solar cell 160, also known as aninterdigitated back contact cell, includes an ARC layer 110, a baseregion 130 made of a semiconductor substrate, and doped regions 120 and125 having opposite polarities from each other (e.g., p-type andn-type). Doped regions 120 and 125 are on the back side of cell 160,opposite of ARC layer 110. A non-conducting layer 170 providesseparation between the doped regions 120 and 125, and also completes therole of passivation of the back surface of cell 160. Electrical contacts140 and 150 are interdigitated with each other and make electricalconnections to doped regions 120 and 125, respectively, through holes175 in the passivating layer 170. Although the electrical contacts 140and 150 do not present a shading issue in this back-contact type ofsolar cell, they may still present other issues such as manufacturingyield losses when forming the contacts onto the cell, high materialcosts if using silver for the contacts, or degradation of the cell ifusing copper for the contacts.

Metallization of solar cells typically involves screen printing a silverpaste in the desired pattern of the electrical contacts to be connectedto the cell. In FIG. 1A, the front contacts 140 are configured in alinear pattern of parallel segments. Because the cost of silver can addgreatly to the expense of the solar cell, it is highly desirable toreduce or even eliminate the use of silver. Copper is an attractivealternative to silver because of its high electrical conductivity, butcan lead to contamination of the semiconductor materials andconsequently reduced performance of the solar cell. Known methods ofutilizing copper in solar cells involve depositing copper directly ontothe cell. However, these methods require subjecting the solar cells tothe temperatures and chemicals involved with the many steps during theseplating processes, which can cause damage to the cell. In other knownmethods, arrangements of parallel copper wires or woven grids of wiresare produced separately from the cell, and then joined to the cell.However, with these methods it can be difficult to align the wires tothe cell, or to produce wires small enough to be functional but yetminimize shading on a solar cell. Wire grids encapsulated withinpolymeric films have also been produced, but these methods can becomplex and still present shading and alignment problems, particularlydue to the presence of the polymeric sheet. Copper paste is anotheralternative, but these pastes can be difficult to apply and stillpresent the problem of diffusion into the solar cell.

In the present disclosure, electrical conduits for semiconductors, suchas photovoltaic cells, are fabricated as an electroformed free-standingmetallic article. The metallic articles are produced separately from asolar cell and can include multiple elements such as fingers and busbars that can be transferred stably as a unitary piece and easilyaligned to a semiconductor device. The elements of the metallic articleare formed integrally with each other in the electroforming process. Themetallic article is manufactured in an electroforming mandrel, whichgenerates a patterned metal layer that is tailored for a solar cell orother semiconductor device. For example, the metallic article may havegrid lines with height-to-width aspect ratios that minimize shading fora solar cell. The metallic article can replace conventional bus barmetallization and ribbon stringing for cell metallization, cell-to-cellinterconnection and module making. The ability to produce themetallization layer for a photovoltaic cell as an independent componentthat can be stably transferred between processing steps provides variousadvantages in material costs and manufacturing.

FIG. 2 depicts a perspective view of a portion of an exemplaryelectroforming mandrel 200 in one embodiment. The mandrel 200 may bemade of electrically conductive material such stainless steel, copper,anodized aluminum, titanium, or molybdenum, nickel, nickel-iron alloy(e.g., Invar), copper, or any combinations of these metals, and may bedesigned with sufficient area to allow for high plating currents andenable high throughput. The mandrel 200 has an outer surface 205 with apreformed pattern that comprises pattern elements 210 and 212 and can becustomized for a desired shape of the electrical conduit element to beproduced. In this embodiment, the pattern elements 210 and 212 aregrooves or trenches with a rectangular cross-section, although in otherembodiments, the pattern elements 210 and 212 may have othercross-sectional shapes. The pattern elements 210 and 212 are depicted asintersecting segments to form a grid-type pattern, in which sets ofparallel lines intersect perpendicularly to each other in thisembodiment.

The pattern elements 210 have a height ‘H’ and width ‘W’, where theheight-to-width ratio defines an aspect ratio. By using the patternelements 210 and 212 in the mandrel 200 to form a metallic article, theelectroformed metallic parts can be tailored for photovoltaicapplications. For example, the aspect ratio may be between about 0.01and about 10. In some embodiments, the aspect ratio can be designed tobe greater than 1, such as between about 1 and about 10, or betweenabout 1 and about 5. Having a height greater than the width allows themetal layer to carry enough current but reduce the shading on the cellcompared to, for example, standard circular wires which have an aspectratio of 1, or compared to conventional screen-printed patterns whichare horizontally flat and have aspect ratios less than 1. Shading valuesfor screen-printed metal fingers may be, for example, over 6%. Withmetallic articles having tailored aspect ratios as described herein,shading values of less than 6% may be achieved, such as between 4-6%.Thus, the ability to produce electrical conduits with aspect ratiosgreater than 1 enable minimal aperture loss to a photovoltaic cell,which is important to maximizing efficiency. In embodiments where theelectroformed electrical conduit is used on a back surface of a solarcell, aspect ratios of other values, such as less than 1, may be used.

The aspect ratio, as well as the cross-sectional shape and longitudinallayout of the pattern elements, may be electroformed to meet desiredspecifications such as electrical current capacity, series resistance,shading losses, and cell layout. Any electroforming process can be used.For example, the metallic article may be formed by an electroplatingprocess. In particular, because electroplating is generally an isotropicprocess, confining the electroplating with a pattern mandrel tocustomize the shape of the parts is a significant improvement formaximizing efficiency. Furthermore, although tall yet narrow conduitlines typically would tend to be unstable when placing them on asemiconductor surface, the customized patterns that may be producedthrough the use of a mandrel allows for features such as interconnectinglines to provide stability for these tall but narrow conduits. In someembodiments, for example, the preformed patterns may be configured as acontinuous grid with intersecting lines. This configuration not onlyprovides mechanical stability to the plurality of electroformed elementsthat form the grid, but also enables a low series resistance since thecurrent is spread over more conduits. A grid-type structure can alsoincrease the robustness of a cell. For example, if some portion of thegrid becomes broken or non-functional, the electrical current can flowaround the broken area due to the presence of the grid pattern.

FIGS. 3A-3C are simplified cross-sectional views of exemplary stages inproducing a metal layer piece using a mandrel. In FIG. 3A, a mandrel 200with pattern elements 210 is provided. The mandrel 200 is subjected toan electroforming process, in which electroformed elements 310 areformed within the pattern elements 210 as shown in FIG. 3B. In theembodiment of FIGS. 3A-3C, the pattern elements 210 have been designedwith a higher aspect ratio than those in FIG. 2. The electroformedelements 310 may be, for example, copper only, or in other embodiments,alloys of copper. In other embodiments, a layer of nickel may be platedonto the mandrel 200 first, followed by copper so that the nickelprovides a barrier against copper contamination of a finishedsemiconductor device. An additional nickel layer may optionally beplated over the top of the electroformed elements 310 to encapsulate thecopper, as depicted by nickel layer 315 in FIG. 3B. In otherembodiments, multiple layers may be plated within the pattern elements210, using various metals as desired to achieve the necessary propertiesof the metallic article to be produced.

In FIG. 3B the electroformed elements 310 are shown as being formedflush with the outer surface 205 of mandrel 200. Electroformed element312 illustrates another embodiment in which the elements may beoverplated. For electroformed element 312, electroplating continuesuntil the metal extends above the surface 205 of mandrel 200. Theoverplated portion, which typically will form as a rounded top due tothe isotropic nature of electroforming, may serve as a handle tofacilitate the extraction of the electroformed element 312 from mandrel200. The rounded top of electroformed element 312 may also provideoptical advantages in a photovoltaic cell by, for example, being arefractive surface to aid in light collection. In yet other embodimentsnot shown, a metallic article may have portions that are formed on topof the surface 205, such as a bus bar, in addition to those that areformed within the preformed patterns 210.

In FIG. 3C the electroformed elements 310 are removed from the mandrel200 as a free-standing metallic article 300. The electroformed elements310 may include intersecting elements 320, such as would be formed bypatterns 212 of FIG. 2. The intersecting elements 320 may assist inmaking the metallic article 300 a unitary, free-standing piece such thatit may be easily transferred to other processing steps while keeping theindividual elements 310 and 320 aligned with each other. The additionalprocessing steps may include coating steps for the free-standingmetallic article 300 and assembly steps to incorporate it into asemiconductor device. By producing the metal layer of a semiconductor asa free-standing piece, the manufacturing yields of the overallsemiconductor assembly will not be affected by the yields of the metallayer. In addition, the metal layer can be subjected to temperatures andprocesses separate from the other semiconductor layers. For example, themetal layer may be undergo high temperature processes or chemical bathsthat will not affect the rest of the semiconductor assembly.

After the metallic article 300 is removed from mandrel 200 in FIG. 3C,the mandrel 200 may be reused to produce additional parts. Being able toreuse the mandrel 200 provides a significant cost reduction compared tocurrent techniques where electroplating is performed directly on a solarcell. In direct electroplating methods, masks or mandrels are formed onthe cell itself, and thus must be built and often destroyed on everycell. Having a reusable mandrel reduces processing steps and saves costcompared to techniques that require patterning and then plating asemiconductor device. In other conventional methods, a thin printed seedlayer is applied to a semiconductor surface to begin the platingprocess. However, seed layer methods result in low throughputs. Incontrast, reusable mandrel methods as described herein can utilizemandrels of thick metal which allow for high current capability,resulting in high plating currents and thus high throughputs. Metalmandrel thicknesses may be, for example, between 0.2 to 5 mm.

FIGS. 4-5 are cross-sectional views of exemplary mandrels, demonstratingembodiments of various mandrel and pattern designs. In FIG. 4, a planarmetal mandrel base 420 has a dielectric layer 430 laid over it. Thepattern including pattern elements 410 for forming a metallic articleare created in dielectric layer 430. The dielectric layer 430 may be,for example, a fluoropolymer (e.g., Teflon®), a patterned photoresist(e.g., Dupont Riston® thick film resist), or a thick layer ofepoxy-based photoresist (e.g., SU-8). The photoresist is selectivelyexposed and removed to reveal the desired pattern. In other embodiments,the dielectric layer 430 may be patterned by, for example, machining orprecision laser cutting. In this type of mandrel 400 withdielectric-surrounded pattern elements, electroplating will fill thetrenches of pattern elements 410 from the bottom up, starting at themetal mandrel base 420. The use of dielectrics or permanent resistsallows for reuse of the mandrel 400, which reduces the number of processsteps, consumable costs, and increases throughput of the overallmanufacturing process compared to consumable mandrels.

FIG. 5 shows another mandrel 500 made primarily of metal, including thecavities for forming a metallic article. When electroforming with metalmandrel 500, the metal surfaces of a pattern element 510 allow for rapidplating from all three sides of the trench pattern. In some embodimentsof mandrel 500, a release layer 520 such as a dielectric or low-adhesionmaterial (e.g., a fluoropolymer) may optionally be coated onto themandrel 500, in various areas as desired. The release layer 520 mayreduce adhesion of the electroformed part to the mandrel 500, or mayminimize adhesion of a substrate, such as an adhesive film, that may beused to peel the electroformed article from the mandrel. The releaselayer 520 may be patterned simultaneously with the metal mandrel, or maybe patterned in a separate step, such as through photoresist with wet ordry etching. The pattern elements 510, 530 and 540 in the metal mandrel,may be, for example, grooves and intersecting trenches, and may beformed by, for instance, machining, laser cutting, lithography, orelectroforming. In other embodiments, the mandrel 500 may not require arelease layer 520 if the surface of the mandrel that is exposed to theplating solution is selected to have poor adhesion to the metallicarticle. For instance, for electroformed parts that will have a firstlayer (that is, an outer layer) of nickel plating, the mandrel 400 maybe made of copper. Copper has low adhesion to nickel and thereby allowsthe formed, nickel-coated piece to be easily removed from the coppermandrel. When applying a release layer 520 to mandrel 500, the relativedepth of the trench pattern element 510 in the metal and the thicknessof the dielectric coating can be selected to minimize void formation ofthe metal piece formed within pattern element 510, while still enablinga high plating rate.

FIG. 5 shows a further embodiment in which the release layer 520 hasbeen extended partially into the depth of pattern element 530. Thisextension of the coating into pattern element 530 may enableelectroforming rates between that of dielectrically-surrounded patternelement 410 of FIG. 4 and metal-surrounded pattern element 510 of FIG.5. The amount that release layer 520 extends into the pattern element530 may be chosen to achieve a desired electroforming rate. In someembodiments, release layer 520 may extend into pattern element 530 by,for example, approximately half the amount of the pattern width. Apattern element 530 with release layer 520 extending into the trench canallow a more uniform electroplating rate within the trench, and hence, amore uniform grid can be produced. The amount that the dielectric orrelease layer 520 extends into the trench can be modified to optimizeoverall plating rate and plating uniformity.

FIG. 5 shows yet another embodiment of mandrel 500 in which the patternelement 540 has tapered walls. The tapered walls are wider at the outersurface 505 of mandrel 500, to facilitate removal of a formed metallicelement from the patterned mandrel. In other embodiments not shown, thecross-sectional shape of the preformed patterns for any of the mandrelsdescribed herein may include shapes such as, but not limited to, curvedcross-sections, beveled edges at the corners of a pattern'scross-section, curved paths along the length of a pattern, and segmentsintersecting each other at various angles to each other.

FIGS. 6A and 6B illustrate top views of exemplary metal layers 600 a and600 b that may be produced by the electroforming mandrels describedherein. Metal layers 600 a and 600 b include electroformed elementsembodied here as substantially parallel fingers 610, which have beenformed by substantially parallel grooves in an electrically conductivemandrel. Metal layer 600 b also includes electroformed elements embodiedhere as horizontal fingers 620 that intersect vertical fingers 610,where the fingers 610 and 620 intersect at approximately a perpendicularangle. In other embodiments, fingers 610 and 620 may intersect at otherangles, while still forming a continuous grid or mesh pattern. Metallayers 600 a and 600 b also include a frame element 630 which may serveas a bus bar to collect current from the fingers 610 and 620. Having abus bar integrally formed as part of the metallic article can providemanufacturing improvements. In present high-volume methods of solarmodule production, cell connections are often achieved by manuallysoldering metal ribbons to the cells. This commonly results in broken ordamaged cells due to manual handling and stress imparted on the cells bythe solder ribbons. In addition, the manual soldering process results inhigh labor-related production costs. Thus, having a bus bar or ribbonalready formed and connected to the metallization layer, as is possiblewith the electroformed metallic articles described herein, enableslow-cost, automated manufacturing methods.

Frame element 630 may also provide mechanical stability such that metallayers 600 a and 600 b are unitary, free-standing pieces when removedfrom a mandrel. That is, the metal layers 600 a and 600 b are unitary inthat they are a single component, with the fingers 610 and 620 remainingconnected, when apart from a photovoltaic cell or other semiconductorassembly. Frame element 630 may furthermore assist in maintainingspacing and alignment between finger elements 610 and 620 for when theyare to be attached to a photovoltaic cell. Frame element 630 is shown inFIGS. 6A-6B as extending across one edge of metal layers 600 a and 600b. However, in other embodiments, a frame element may extend onlypartially across one edge, or may border more than one edge, or may beconfigured as one or more tabs on an edge, or may reside within the griditself. Furthermore, frame element may be electroformed at the same timeas the fingers 610 and 620, or in other embodiments may be electroformedin a separate step, after fingers 610 and 620 have been formed.

FIG. 6C shows a cross-section of metal layer 600 b taken at section B-Bof FIG. 6B. Fingers 610 in this embodiment are shown in as having aspectratios greater than 1, such as about 1 to about 5, and such asapproximately 2 in this figure. Having a cross-sectional height greaterthan the width reduces the shading impact of metal layer 600 b on aphotovoltaic cell. In various embodiments, only a portion of the fingers610 and 620 may have an aspect ratio greater than 1, or a majority ofthe fingers 610 and 620 may have an aspect ratio greater than 1, or allof the fingers 610 and 620 may have an aspect ratio greater than 1.Height ‘H’ of fingers 610 may range from, for example, about 5 micronsto about 200 microns, or about 10 microns to about 300 microns. Width‘W’ of fingers 610 may range from, for example, about 10 microns toabout 5 mm, such as about 10 microns to about 150 microns. The distancebetween parallel fingers 610 has a pitch ‘P’, measured between thecenterline of each finger. In some embodiments the pitch may range, forexample, between about 1 mm and about 25 mm. In FIGS. 6B and 6C, thefingers 610 and 620 have different widths and pitches, but areapproximately equivalent in height. In other embodiments, the fingers610 and 620 may have different widths, heights and pitches as eachother, or may have some characteristics that are the same, or may haveall the characteristics the same. The values may be chosen according tofactors such as the size of the photovoltaic cell, the shading amountfor a desired efficiency, or whether the metallic article is to becoupled to the front or rear of the cell. In some embodiments, fingers610 may have a pitch between about 1.5 mm and about 6 mm and fingers 620may have a pitch between about 1.5 mm and about 25 mm. Fingers 610 and620 are formed in mandrels having grooves that are substantially thesame shape and spacing as fingers 610 and 620. Frame element 630 mayhave the same height as the fingers 610 and 620, or may be a thinnerpiece as indicated by the dashed line in FIG. 6C. In other embodiments,frame element 630 may be formed on above finger elements 610 and 620.

FIG. 6C also shows that fingers 610 and 620 may be substantiallycoplanar with each other, in that the fingers 610 and fingers 620 have amajority of their cross-sectional areas that overlap each other.Compared to conventional meshes that are woven over and under eachother, a coplanar grid as depicted in FIG. 6C can provide a lowerprofile than overlapping circular wires of the same cross-sectionalarea. The intersecting, coplanar lines of metal layer 600 b are alsoformed integrally with each other during the electroforming process,which provides further robustness to the free-standing article of metallayer 600 b. That is, the integral elements are formed as one piece andnot joined together from separate components. FIGS. 6D and 6E show otherembodiments of coplanar, intersecting elements. In FIG. 6D, finger 610is shorter in height than 620 but is positioned within thecross-sectional height of finger 620. Fingers 610 and 620 have bottomsurfaces 612 and 622, respectively, that are aligned in this embodiment,such as to provide an even surface for mounting to a semiconductorsurface. In the embodiment of FIG. 6E, finger 610 has a larger heightthan finger 620 and extends beyond the top surface of finger 620. Amajority of the cross-sectional area of finger 610 overlaps the entirecross-section of finger 620, and therefore fingers 610 and 620 arecoplanar as defined in this disclosure.

FIGS. 6F and 6G show yet other embodiments, in which electroformedmetallic articles enable interconnections between photovoltaic cells ina module. A typical module has many cells, such as between 36-60,connected in series. The connections are made by attaching the front ofone cell to the back of the next cell using solder-coated copper ribbon.Attaching the ribbon in this way requires a ribbon that is thin, andconsequently resistive, so that the ribbon can bend around the cellswithout break the cell edges. The interconnections also typicallyrequire three separate ribbons, each soldered separately. In theembodiment of FIG. 6F, a metallic article 650 has interconnectionelements 660 that have been integrally electroformed with a first gridregion 670. Interconnection elements 660 have a first end coupled togrid 670, and are configured to extend beyond the surface of aphotovoltaic cell to allow connection to a neighboring cell. Theinterconnection elements 660 replace the need for a separate ribbon tobe soldered between cells, thus reducing manufacturing costs andenabling possible automation. In the embodiment shown, interconnectionelements 660 are linear segments, although other configurations arepossible. Also, the number of interconnection elements 660 can vary asdesired, such as providing multiple elements 660 to reduce resistance.Interconnection elements 660 may be bent or angled after electroforming,such as to enable a front-to-back connection between cells, or may befabricated in the mandrel to be angled relative to the grid 670.

The opposite end of interconnection elements 660 may be coupled to asecond region 680, where the second region 680 may also be electroformedin an electrically conductive mandrel as part of the metallic article650. In FIG. 6F, the second region 680 is configured as a tab—e.g., abus bar—that may then be electrically connected to an electrical conduit690 of a neighboring cell. The conduit 690 is configured here as a mesh,but other configurations are possible. Grid 670 may, for example, serveas an electrical conduit on a front surface of a first cell, while grid690 may be an electrical conduit on a rear surface of a second cell. Inthe embodiment of FIG. 6G, a metallic article 655 has a mesh instead ofa bus bar type of connection. Metallic article 655 includes first region670, interconnection elements 660 and second region 690 that have allbeen electroformed as a single component, such that the inter-cellconnections are already provided by metallic article 655. Thus themetallic articles 650 and 655 provide electrical conduits not only on asurface of one photovoltaic cell, but also the interconnections betweencells.

Although the mandrels described in FIGS. 2-5 have been described as flatmandrels, the mandrel may instead be cylindrical to be conducive to acontinuous process. FIG. 17 shows a cross-sectional view of an exemplarycylindrical mandrel 1700, with preformed pattern 1710 created on outersurface 1720. In such embodiments, the cylindrical mandrel 1700 may bedipped and rotated in an electroforming bath, and the resulting unitarymetallic article may be produced as a continuous strip that can later betrimmed to into separate, unitary pieces as needed. In otherembodiments, a flat mandrel 1800, exemplified in the cross-sectionalview of FIG. 18, may have a first preformed pattern 1810 in a topsurface 1820 and a second preformed pattern 1850 in a bottom surface1860. The first and second preformed patterns 1810 and 1850 may be thesame or different from each other. For example, in FIG. 18 the firstpreformed pattern 1810 has elements with different width, height andpitch than the second preformed pattern 1850. The two-sided mandrel 1800may be used to produce the two patterns at once, or in otherembodiments, one side may be masked while the other side is used toproduce an electroformed part. In one embodiment, the first preformedpattern may be used to produce a metallic article for the front side ofa solar cell, and the second preformed pattern may be used to form ametallic article for the back side of the solar cell.

FIG. 7 depicts an exemplary flow chart 700 for fabricating afree-standing electroformed metallic article for use with asemiconductor assembly such as a photovoltaic cell. In this disclosure,reference to semiconductor materials in formation of a semiconductordevice or photovoltaic cell may include amorphous silicon, crystallinesilicon or any other semiconductor material suitable for use in aphotovoltaic cell. In a step 710, an electroforming process is performedusing an electrically conductive mandrel. The mandrel has one or morepreformed patterns in which to form a metallic article. In someembodiments, the metallic article is configured to serve as anelectrical conduit within a photovoltaic cell. In certain embodiments,the metallic article may include features to enable connections betweenphotovoltaic cells of a solar module. The preformed pattern may have anaspect ratio of greater than 1, and may include multiple parallelpatterns intersecting each other. At least a portion of the finishedelectroformed metallic article is created within the preformed patterns.Other portions of the metallic article, such as a bus bar, may be formedwithin preformed patterns or on a top surface of the mandrel.

The electroforming step 710 may include contacting the outer surface ofthe mandrel with a solution comprising a salt of a first metal, wherethe first metal may be, for example copper or nickel. The first metalmay form the entire metallic article, or may form a metallic precursorfor layers of other metals. For example, a solution of a salt comprisinga second metal may be plated over the first metal. In some embodiments,the first metal may be nickel and the second metal may be copper, wherethe nickel provides a barrier for copper diffusion. A third metal mayoptionally be plated over the second metal, such as the third metalbeing nickel over a second metal of copper, which has been plated over afirst metal of nickel. In this three-layer structure, the copper conduitis encapsulated by nickel to provide a barrier against coppercontamination into a semiconductor device. Electroforming processparameters in step 710 may be, for example, currents ranging from 1 to3000 amps per square foot (ASF) and plating times ranging from, forexample, 1 minute to 200 minutes. Other electrically conductive metalsmay be applied to promote adhesion, promote wettability, serve as adiffusion barrier, or to improve electrical contact, such as tin, tinalloys, indium, indium alloys, bismuth alloys, nickel tungstate, orcobalt nickel tungstate.

After the metallic article is formed, the metallic article is separatedin step 720 from the electrically conductive mandrel to become afree-standing, unitary piece. The separation may involve lifting orpeeling the article from the mandrel, with or without the use of atemporary polymeric sheet, or with or without the use of vacuumhandling. In other embodiments, removal may include thermal ormechanical shock or ultrasonic energy to assist in releasing thefabricated part from the mandrel. The free-standing metallic article isthen ready to be formed into a photovoltaic cell or other semiconductordevice, by attaching and electrically coupling the article as shall bedescribed below. Transferring of the metallic article to the variousmanufacturing steps may be done without need for a supporting element,such as a plastic or polymeric substrate, which can reduce cost.

The free-standing metallic article may be mounted directly to a solarcell or may undergo additional processing steps prior to being attached.Note that for the purposes of this disclosure, the term “metallicarticle” may also be interchangeably referred to as a grid or mesh, eventhough some embodiments may not include intersecting cross-members. Ifthe metallic article has been formed without a barrier layer, theseparated, free-standing metallic article may optionally undergoadditional plating operations in step 730. For example, nickel platingmay be performed by, for example, electroless or electroplating. In someembodiments, the metallic article may also be plated withnickel-cobalt-tungsten or cobalt-tungsten-phosphorous to create adiffusion barrier for copper material at high temperatures, while thestandard nickel plating prevents copper migration in the cell below 300°C.

After any additional plating has been completed, in step 740 anattachment mechanism may be applied to the free-standing metallicarticle to prepare it for being mounted to a cell surface. For astandard solar cell model, a reactive metal layer such as a fire-throughsilver paste may be applied to the surface of the metallic article thatis to be coupled to the solar cell. The reactive paste provides theelectrical connection between the metallic article and the semiconductorlayer, and may be thinly applied. The paste may be applied to theelectroformed metallic article by, for example, screen printing. Theamount of silver that is applied to the grid is much less than thatwhich is required when forming the metallization layer solely fromfire-through paste. Because the fire-through paste is applied onto thegrid rather than the solar cell, the electrical coupling between thegrid and solar cell is self-aligned. That is, there is no need to alignthe fingers of the electrical conduit to conductive lines of paste thathave been applied onto the solar cell, thus simplifying themanufacturing process. Furthermore, in conventional methods, extra pasteis often applied to ensure alignment with electrical contacts. Incontrast, the present methods enable the application of silver pasteonly where necessary. Additional methods of applying the attachmentmechanism include electroplating; electroless plating; wave soldering;physical vapor deposition techniques such as evaporation or sputtering;dispensing via ink jet or pneumatic dispensing techniques; or thin filmtransfer techniques such as stamping the grid onto a thin film of moltensolder or metal.

While some types of solar cells use dielectric ARC's, other types useconductive ARC's, such as TCO's. For TCO types of solar cells, such asthose coated with indium-tin-oxide (ITO), the attachment mechanism instep 740 may be solder, such as a low temperature solder. The solder isapplied to the surface of the grid that will be in contact with thecell. By applying solder to the grid, a minimal amount of solder isused, thus reducing material cost. In addition, the solder isself-aligned with the grid pattern. The type of solder on the metallicarticle may be chosen for characteristics such as good ohmic contact andelectrical conductivity, strong adhesion, rapid thermal dissipation, lowcoefficient of thermal expansion (CTE) mismatch with the targetedsurface, robust mechanical stress relief, high mechanical strength,solid electrical migration barrier, adequate wettability, and chemicallysound material inter-diffusion barriers between the metallicelectroformed grid and the surface of the solar cell. In one embodiment,a no-clean solder may be applied. In another embodiment, an electrolessor electroplated low melting point metal or alloy—such as, but notlimited to, indium, indium-tin, indium-bismuth, lead-tin-silver-copper,lead-tin-silver, and lead-indium—may be applied to the grid. In afurther embodiment, a solder paste may be printed onto the grid. Thesolder paste may require a drying process before the grid and the solarcell can be coupled together. In yet another embodiment, the tips—thatis, the bottom surface—of the grid may be dipped or immersed into aliquid solder, which will selectively attach to the mesh surface.

Although the attachment mechanisms above have been described as beingapplied to the electroformed article, in other embodiments, step 740 mayinclude applying the fire-through paste or solder material to the solarcell. The electroformed article would then be brought into contact withthe conductive patterns made by the paste or solder. The metallicarticle may be prepared for contacting with the cell by optionallyapplying an indium metal or indium alloy to the article. The indium canbe electroplated onto the surface of the grid by dipping the grid intothe electrolyte while providing current. In another embodiment, the gridmay be coated by an electroless plating method by dipping it into asolution of indium. The grid can be dipped first into a molten flux,which removes oxide on the tips of the grid, and then into an indium tinsolder such that only the tips of the grid are wetted with the indiumtin solder. In another embodiment, the grid can be dipped into indiumtin paste followed by an anneal step, again with only the tips of thegrid being coated. Coating of only the tip, and not the entire grid,with indium preserves precious indium while still achieving acontactable surface. Once indium-tipped, the fingers or elements of theelectroformed article may then be aligned with the fire-through paste orsolder on the cell by, for example, optical alignment marks on edges ofthe solar cell.

In further embodiments, the metallic articles may be utilized inback-contact types of solar cells, such as those illustrated in FIG. 1B,using similar methods. An attachment mechanism, which would typically besolder, is applied to either the metallic article or the solar cell instep 740, and the metallic article is then contacted with the cell. Theattachment mechanism is heated to electrically couple the metallicarticle with the cell. In one embodiment of back-contact solar cells,the electroformed elements of a first metallic article would be coupledto the p-type regions on the rear surface of the cell, while theelectroformed elements of a second metallic article would be coupled tothe n-type regions. For example, the metallic articles could beconfigured with linear fingers, as in FIG. 6A, and the fingers of thefirst metallic article would be interdigitated with the fingers of thesecond metallic article.

After an attachment mechanism has been applied to the metallic article,the metallic article is coupled to the cell or semiconductor devicesurface in step 750. The metallic article is brought into contact withthe surface of the solar cell. If the grid article has been tipped withfire-through silver paste, the assembly is heated to the fire-throughtemperature of the paste, such as to temperatures of at least 400° C.,or at least 800° C. The grid may be held mechanically stable duringfiring by the use of rollers or clamps. Once the fire-through paste isset, neighboring solar cells in a module may be interconnected. Forsolder-tipped grids, the grid is similarly coupled to the solar cell andheated to temperatures required for the particular solder typicallyranging between 100° C. and 300° C. A thermal and/or pressure process inatmosphere or vacuum may be used to reflow the solder and form thecontacts between the metallic article and the solar cell.

In some embodiments, the independent grid or metallic article, afterbeing plated with the desired barrier layers, can be attached to a solarcell prior to anti-reflective coating layer deposition. In a standardcell, the grid can be contacted to the emitter surface (e.g., dopedsilicon) and heated to create a nickel silicide chemical bond. The ARC,such as a nitride, can then be deposited after grid attachment, inoptional step 760. A bus bar of the grid can then be connected toanother cell in the module. This embodiment of attaching the grid beforethe ARC layer eliminates the need for any silver fire-through usage. Inaddition, this embodiment may be applied to silicon heterojunction solarcells. For instance, the free-standing metallic article, such as a grid,can be coupled to the surface of the heterojunction cell amorphoussilicon layer. It can then be heated to create a nickel silicide bond,and the ITO layer can be deposited on the grid afterwards.

After the completed photovoltaic cell has been formed in step 750, themultiple cells that form a solar module may be interconnected in step770. In some embodiments, the bus bars or tabs that have beenelectroformed as part of the metallic article may be utilized for theseinterconnections.

It can be seen that the free-standing electroformed metallic articledescribed herein is applicable to various cell types and may be insertedat different points within the manufacturing sequence of a solar cell.Furthermore, the electroformed electrical conduits may be utilized oneither the front surface or rear surface of a solar cell, or both. Whenelectroformed articles are used on both front and back surfaces, theymay be applied simultaneously to avoid any thermal expansion mismatchwhich may cause mechanical bending of the cells.

FIGS. 8A-8B illustrate a schematic of an exemplary photovoltaic cell 800produced with a free-standing metallic article 810. Metallic article 810in this embodiment includes electroformed elements 812 and a frameelement 814 that spans an edge near the perimeter of the electroformedelements 812. Electroformed elements 812 are shown as parallel linesthat intersect perpendicularly in this embodiment to form a continuousgrid pattern, but in other embodiments they may be configured with linesintersecting at other angles, or as one set of parallel lines, or asother patterns. The tips of electroformed elements 812 have anattachment material 820, such as solder or fire-through silver paste,applied to them. The attachment material 820 electrically couples themetallic article 810 to a photovoltaic component 830, where thephotovoltaic component 830 may include light incident layer 832 (e.g.,ARC and/or TCO), active region 834 (emitter and base), and rear contactlayer 836. FIG. 8B shows another embodiment of a photovoltaic cell 800in which layer 832 is an ARC, in which the attachment material 820 is asilver paste that has been fired through the ARC. In FIGS. 8A-8B, anencapsulant (not shown) may be applied over metallic article 810 to sealthe completed photovoltaic cell 800, with interconnection with othercells being made with the frame element 814. In other embodiments, asecond metallic article 810 may be similarly coupled to rear contactlayer 836, which is a non-incident light surface, to provide anelectrical contact of opposite polarity for the photovoltaic cell 800.

FIGS. 8C-8D show simplified schematics of an exemplary back-contactsolar cell 801 produced with free-standing metallic articles. In thecross-sectional view of FIG. 8C, solar cell 801 includes transparentlayer 831 (e.g., an ARC), semiconductor substrate 833, doped regions 835and 837, and passivating layer 840. Two free-standing metallic articles850 and 860 have electroformed elements that are positioned in analternating fashion. The electroformed elements of metallic articles 850and 860 provide electrical contact with doped regions 835 and 837,respectively, through the holes 845 in passivating layer 840. FIG. 8Dshows a top view of metallic articles 850 and 860 used in solar cell801. Metallic article 850 has fingers 852 that are interdigitated withfingers 862 of metallic article 860. Frame elements 854 of metallicarticle 850 and frame element 864 of metallic article 860 serve as anelectrical connection point for each metallic article, and also providemechanical stability.

FIGS. 9A-9B illustrate yet another embodiment in which the shadingimpact of solder applied between the metallic article and solar cell canbe reduced. FIG. 9A shows a vertical cross-sectional view of a standardsolder joint 910 that may result from soldering a metal element 920,having a rectilinear cross-section, to a solar cell 930. Because soldernaturally forms a wetting angle between the surfaces that it is joining,the solder has a footprint with a width ‘F1’. The width of thisfootprint will block light from entering the solar cell 930, and thuscauses shading. In FIG. 9B, the cross-sectional shape of electroformedelement 925 has been altered compared to electroformed element 920, inthat electroformed element 925 has chamfered corners 927 on its lowersurface. The chamfering changes the wetting angle of the solder joint910, such that the footprint width ‘F2’ is less than ‘F1’. Thus, thetailored shape of electroformed element 925 reduces shading. The abilityto customize the cross-sectional shape of electroformed element 925 ismade possible by the use of an electroforming mandrel, as described inthe various embodiments above. Features such as chamfering, filleting,dimples, nubs, and the like may be formed in the mandrel to impart thesefeatures to the electroformed part that is to be produced.

FIGS. 10A-10B show top views of another embodiment of reducing theshading impact from solder applied to a metallic article. FIG. 10A showsa conventional solder joint 1010 applied to two obliquely intersectinglinear segments 1020. The total footprint of the solder joint 1010 has awidth ‘F3’. FIG. 10B shows electroformed elements 1025 where concavecut-out features 1027 have been incorporated into the corners where theelectroformed elements 1025 intersect, through features of the mandrelin which the elements 1025 have been formed. The concave features 1027changes the wetting angle of the solder 1010, such that the footprintwidth ‘F4’ is reduced compared to ‘F3’. Shapes other than the concavefeatures shown here are possible. Thus, the ability to tailor the shapeof the electroformed elements, by incorporating features into a formingmandrel, can reduce the shading impact of the solder that is used tocouple the electroformed elements to a photovoltaic surface.

In another embodiment shown in FIGS. 11A-11C, a portion of the mandrelin which the metallic article is formed may become part of a finalsemiconductor device. FIG. 11A shows a cross-sectional view of a mandrel1100 similar to previously described mandrel 400 of FIG. 4, having ametal base 1120 and a dielectric layer 1130 with patterns for formingelectroformed elements 1110. Electroformed elements 1110 have beenformed in dielectric layer 1130 during the electroforming process. Inaddition, the plating thickness may also exceed the height of themandrel patterns to form overplated heads 1112. In other embodiments, nooverplating is performed, as in electroformed elements 1111. Whenremoving the metallic article comprising electroformed elements 1130from the mandrel 1100, dielectric layer 1130 may be peeled off, alongwith the electroformed elements 1130, from mandrel metal base 1120 asindicated by arrow 1140. The heads 1112 may help secure theelectroformed elements 1110 to the dielectric layer 1130.

In FIG. 11B the separated metallic article 1150, which is a combinationof electroformed elements 1110 and is surrounded by the dielectric layer1130, may then be coupled to a semiconductor surface to form, forexample, a photovoltaic cell. One embodiment of a solar cell 1160 isdepicted in the simplified schematic of FIG. 11C. Solar cell 1160includes a semiconductor assembly 1170. Metallic article 1150 is coupledto semiconductor assembly 1170, and is overlaid by an encapsulant 1180and a window layer 1190 such as an anti-reflective coating. Encapsulant1180 may be, for example, ethylene vinyl acetate (EVA), thermoplasticpolyolefin (TPO) or polyvinyl butyral (PVB). The dielectric layer 1130of FIGS. 11A-11C can be chosen to be suitable for the appropriatesemiconductor application. For a photovoltaic cell, the targetcharacteristics of the transferable dielectric will depend on thereliability specifications of the intended solar module. Because thedielectric will be incorporated into the module, it must have adurability to withstand the lifetime of a solar module. The dielectricmust also be transparent to allow light to be transmitted to the solarcell, and should also be resistant to copper diffusion into the cell.One type of suitable dielectric is, for example solder resistantdielectrics that are known in the electronic packaging industry.

In other embodiments, the metallic article described herein may becombined with a polymer sheet to form a polymer layer. FIG. 12 shows oneembodiment of such a method, in which a metallic article havingelectroformed elements 1210 has been formed with a mandrel 1220.Electroformed elements 1210 may be configured, for example, as a set ofparallel lines, or sets of intersecting lines forming a grid. For thisembodiment, electroformed elements 1210 have been overplated to form arounded head 1212 at their top surface, as has been described above inrelation to electroformed element 312 of FIG. 3B. A polymer sheet 1230is placed over the surface of the mandrel and is used to remove theelectroformed elements 1210 from the mandrel 1220. FIG. 12 shows a statein which the polymer sheet 1230 and electroformed elements 1210 havebeen lifted from the mandrel 1220. The polymer sheet 1230 is contactedto the mandrel such that the heads 1212 of electroformed elements 1210are at least partially embedded into the polymer sheet 1230. The heads1212 enable the polymer sheet 1230 to grip the electroformed elements1210 because of the larger surface area, and the heads 1212 also mayserve as anchor points. Note that although the heads 1212 are embodiedwith curved surfaces, other shapes are possible. In addition, for somemetallic articles and mandrels, overplating may not be needed. Thepolymer sheet 1230 with the embedded heads 1212 of electroformedelements 1210 is lifted from the mandrel 1220, which pulls the heads1212 upward, which in turn lifts the electroformed elements 1210 of themetallic article off the mandrel 1220. The bottom of the electroformedelements 1212 remains exposed from the polymer sheet 1230, hanging fromthese anchor points, which allows them to be subsequently coated orplated as needed.

The polymer sheet 1230 may be made of, for example, EVA, TPO or PVB.Polymer sheet 1230 may optionally be structured as a substrate layer1232 covered by an adhesive layer 1234. The adhesive layer 1234 facesthe mandrel, to engage the electroformed elements 1210. The substratelayer 1232 may be, for example, polyethyelene, polyester or polyesterfilms (e.g., Mylar®) and the adhesive layer 1234 may be, for example,EVA or TPO. If the polymer sheet 1230 includes an adhesive, mandrel 1220may include an optional release layer 1225 to allow the polymer sheet1230 to be easily peeled from the mandrel 1220. Release layer 1225 maybe, for example, a fluoropolymer, or other low-adhesion materials. Theadhesive layer 1234 is made with a thickness to enable the heads 1212 tobe at least partially embedded in it.

In some embodiments, the polymer sheet 1230 is used primarily to removethe electroformed elements 1212 from the mandrel, such as to serve as atransfer material. The polymer sheet 1230 can then be separated from theelectroformed elements 1212, resulting in a free-standing metallicarticle as has been described in previous embodiments. Using a polymersheet to remove the metallic article from the electrically conductivemandrel can make the processing conducive to automation, which enableshigh throughputs. The polymer sheet can also provide support for theelectroformed metallic article while the article undergoes additionalmanufacturing steps. For example, because the bottom surfaces of theelectroformed elements 1210 remain exposed after being extracted frommandrel 1220, the polymer sheet 1230 may be used to hold the metallicarticle while the bottom surfaces are, for example, plated with barrierlayers or applied with solder or fire-through paste. The polymer sheet1230 may also provide additional mechanical support to preserve thedimensions of the grid during handling.

In other embodiments, the polymer sheet may become a component in afinal semiconductor device in which the metallic article is to beplaced. FIGS. 13A-B show an exemplary embodiment, in which a polymerlayer 1315 is placed on a semiconductor component 1370 to form aphotovoltaic cell 1300. In this embodiment, the polymer layer 1315serves as an electrical conduit for the rear surface of the photovoltaiccell 1300. However, the process described for FIGS. 13A-B may also beutilized for the polymer layer 1315 serving as a front contact, or bothfront and rear. Polymer layer 1315 includes polymer sheet 1330 andelectroformed elements 1310, which are similar to polymer sheet 1230 andelectroformed elements 1210 of FIG. 12. The semiconductor component 1370may be, for example, a solar cell with layers such as an active region,rear contact, and TCO layers. In some embodiments, the polymer layer1315 may have a reactive metal layer (not shown) applied to the exposedsurface of electroformed elements 1310, or the reactive metal layer maybe applied to the surface of semiconductor component 1370 that isreceiving the electroformed elements 1310. The polymer layer 1315 ismechanically and electrically coupled to the cell 1370 using heat andpressure. The applied heat and pressure pushes the grid into the polymermaterial 1330, as shown in FIG. 13B. The electroformed elements 1310create mechanical anchor points in the polymer 1330 and provide solidstabilization of the electroformed elements 1310 within polymer layer1315. The polymer material 1330 is chosen to have the necessarycharacteristics of a solar encapsulant material, such as transparency,durability, wettability and corrosion resistance, among otherconstraints which may be necessary depending on the cell type. Thematerial for polymer sheet 1330 may be, for example, EVA, TPO, PVB andionomer.

FIG. 14 is an exemplary flow chart 1400 for using a polymeric substratein combination with an electroformed metallic article, such as a grid ormesh. In step 1410, a metallic article is fabricated by anelectroforming process using an electrically conductive mandrel withpreformed patterns. The metallic article is contacted with a polymersheet in step 1420, where a portion of the metallic article is embeddedwithin the polymer sheet. In step 1430 the polymer sheet andelectroformed elements are lifted or peeled from the mandrel to separatethe polymer layer from the mandrel, where the polymer layer is acomposite of the polymer sheet and the electroformed grid partiallycontained in it. In optional step 1440, additional plating or otherprocesses can be performed on the exposed portions of the electroformedelements. For example, step 1440 may include plating nickel or anotherbarrier material on the exposed portions of the grid, if nickel was notlayered during the electroforming process. Step 1440 may also includecleaning steps, such as to remove oxides to prepare the grid forsoldering.

If the polymer sheet is used primarily as a transfer material, thepolymer sheet may be detached from the metallic article in step 1450.The metallic article can then be processed into a photovoltaic cell orother semiconductor device in step 1460, which may include performingsteps 740 to 770 of FIG. 7. In other embodiments in which the polymersheet is to be incorporated into the finished device, in step 1470 anattachment mechanism may be applied to either the grid or thesemiconductor device, as has been described in step 740 of FIG. 7. Thepolymer layer is then coupled to the semiconductor device, such as bybonding using heat and pressure, in step 1480. This bonding processresults in the polymer material encapsulating the electroformed grid,and also electrically couples any solder or fire-through paste betweenthe grid and the solar cell. The bonding process may include subjectingthe cell and polymer layer to a lamination process with vacuum, elevatedtemperature and pressure. Under the lamination conditions, solderreflows and forms an electrical contact between the cell andpolymer-supported metal grid, while the polymer bonds to the cellsurface and makes a robust mechanical contact. The photovoltaic cell maythen be completed in step 1490 by performing any finishing steps, suchas applying an anti-reflective layer and forming interconnections withother cells in a solar module. The process of flow chart 1400 isapplicable for both front and backside connections, as well as tovarious types of solar cells including standard, non-standardTCO-coated, and back-contact (e.g., interdigitated back contact) cells.

In yet another embodiment, the metallic article disclosed herein may beused as a mask for a conductive layer on a semiconductor surface,wherein the metallic article is consequently self-aligned with thepattern produced on the conductive layer. FIG. 15A shows a perspectiveview of portion of a semiconductor device 1510, which includes layersfor a solar cell. The semiconductor device 1510 has a conductive metallayer 1520 placed on its top surface. Conductive metal layer 1520, whichmay also be referred to in the industry as a contact layer, maysubstantially cover the full surface of semiconductor device 1510. Thesurface that is covered by conductive metal layer 1520 may be a lightincident top surface of a solar cell. Conductive metal layer 1520 maybe, for example a thin film of metal deposited onto a standard solarcell processed just prior to ARC layer deposition or through completionof a fired-through metal layer. Conductive layer 1520 may alternativelybe a TCO layer. In one embodiment, conductive layer 1520 may be a thinlayer of titanium with nickel deposited over it. The conductive metallayer 1520 is chosen to make good ohmic contact to the semiconductordevice 1510, and provide excellent adhesion to the semiconductor device1510 and to the metal grid that shall be subsequently attached.Conductive metal layer 1520 may be, for example, titanium, tungsten,chromium, molybdenum, or combinations thereof, and may be provided onthe semiconductor device using any method known in the art, includingdeposition methods such as physical vapor deposition or electroplating.The thickness of conductive metal layer 1520, in some embodiments, canbe only as thick as necessary to provide a uniform film that canmaintain the required electrical and mechanical properties.

A metallic article, embodied as grid 1530 in FIG. 15B, can bemechanically and electrically coupled to the assembly comprising thesemiconductor device 1510 and conductive metal layer 1520. This coupling(not shown) can be adhesion through the use of a solder paste,electrically conductive adhesive, or conventional solder such that themetal grid 1530 has good electrical and mechanical contact to theconductive metal layer 1520. The solder, solder paste, or adhesive maybe applied to the grid 1530, such as to the bottom surface of grid 1530.This grid 1530 is designed such that it is highly conductive, yetprovides a relatively low amount of shading over the cell. Grid 1530,for example, may have lines with a tall height to provide sufficientconductivity but a narrow width to minimize shading.

The metallic article attached to the conductive metal layer 1520 can beused as a mask to pattern the conductive metal layer 1520, so that thebulk of the solar cell area can be cleared for light absorption. Forexample, as shown, a masked region 1540 is formed directly beneath thegrid 1530, while an exposed portion 1545 comprises the remainingportions of conductive metal layer 1520, where the grid 1530 is absent.The exposed portion 1545 can be removed such that conductive metal layer1520 becomes patterned into the shape of the grid 1530. The conductivemetal layer 1520 can be patterned by, for example, removing exposedportion 1545 with a wet chemical etch process, a dry etch process suchas reactive ion etching, or by a physical etch process such as, but notlimited to, ion milling. The etching process may remove all or a portionof the exposed region 1545.

FIG. 15C shows the assembly after etching, such that only the maskedregion 1540 remains on the surface of semiconductor device 1510 in thisembodiment. The masked region 1540 has a substantially similar patternas grid 1530, and is coincident with grid 1530. Thus, the metal grid1530 provides a chemically resistant mask in the case of wet or reactiveion etching, and a mechanical mask in the case of physical etching,allowing for the coupling and alignment of a separate metallic articleto the semiconductor assembly. In FIG. 15D, a further embodiment isshown in which the semiconductor device 1510 is a standard cell, and inwhich the grid 1530 has been coupled to silicon instead of a TCO. Afteretching, a nitride layer 1550 has been deposited onto the areas thatwere previously occupied by the exposed portions of conductive metallayer 1520, to form an ARC layer for the photovoltaic cell. While notshown, metal grid 1530 may also be coated.

FIG. 16 illustrates an exemplary flow chart 1600 for using a metallicarticle as a mask. In step 1610, a conductive metal layer is provided ona surface of a semiconductor material. In step 1620, a metallic articleis electrically and mechanically coupled to the conductive metal layer.The metallic article may be electroformed in an electrically conductivemandrel having preformed patterns, as has been described above andshown, for example, in FIGS. 2-7. The portions of the surface of thesemiconductor material that are covered by the metallic article aremasked regions, and the uncovered portions are exposed regions. In step1630, the exposed regions are partially or fully removed by, forexample, one of various etching processes as has been described inrelation to FIG. 15B-15C. The resulting assembly, with the conductivemetal layer that is patterned and self-aligned with the metallicarticle, can now be processed further for fabrication into a finishedsemiconductor device assembly such as a solar cell. By using the grid asa mask, the total number of process steps is greatly reduced compared toconventional masking techniques in which a separate masking andpatterning process must be undertaken in order to pattern the contactlayer. Furthermore, the need for alignment between the metal grid andthe conductive lines is eliminated since the mask is self-aligned withthe patterned conductive lines that are produced. The metallic grid alsoprovides an added level of robustness compared to conventionalfired-through silver contacts.

Although the embodiments herein have primarily been described withrespect to photovoltaic applications, the methods and devices may alsobe applied to other semiconductor applications such as redistributionlayers (RDL's) or flex circuits. Furthermore, the flow chart steps maybe performed in alternate sequences, and may include additional stepsnot shown.

While the specification has been described in detail with respect tospecific embodiments of the invention, it will be appreciated that thoseskilled in the art, upon attaining an understanding of the foregoing,may readily conceive of alterations to, variations of, and equivalentsto these embodiments. These and other modifications and variations tothe present invention may be practiced by those of ordinary skill in theart, without departing from the scope of the present invention, which ismore particularly set forth in the appended claims. Furthermore, thoseof ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention.

What is claimed is:
 1. A method of forming an electrical component for aphotovoltaic cell, the method comprising the steps of: i) electroforminga metallic article on an electrically conductive mandrel, wherein theelectrically conductive mandrel has an outer surface comprising at leastone preformed pattern, the preformed pattern comprising a continuousgrid pattern coupled to a cell interconnection pattern, wherein themetallic article comprises a plurality of electroformed elements formedin the preformed pattern, the plurality of electroformed elementscomprising a cell interconnection element coupled to a continuous gridhaving a plurality of first elements intersecting a plurality of secondelements, and wherein the cell interconnection element is configured todirectly couple the grid to a neighboring cell; and ii) separating themetallic article from the electrically conductive mandrel, wherein theplurality of electroformed elements are interconnected such that themetallic article forms a unitary, free-standing piece when separatedfrom the electrically conductive mandrel; wherein the preformed patternhas a height and a width, wherein the ratio of the height to the widthis an aspect ratio, and wherein the preformed pattern has an aspectratio greater than 1; and wherein the plurality of electroformedelements are configured to serve as an electrical conduit for alight-incident surface of a photovoltaic cell.
 2. The method of claim 1,wherein the aspect ratio is between about 1 and about
 10. 3. The methodof claim 1, wherein the step of electroforming comprises overplating theelectroformed elements such that the electroformed elements extend abovethe outer surface of the electrically conductive mandrel.
 4. The methodof claim 1, wherein the step of electroforming the metallic articlecomprises contacting the outer surface of the mandrel with a firstsolution comprising a salt of a first metal, wherein a first metalliclayer comprising the first metal is formed within the preformed pattern.5. The method of claim 1, wherein the plurality of first elements andthe plurality of second elements comprise a plurality of substantiallyparallel first elements intersecting a plurality of substantiallyparallel second elements, wherein the plurality of first elements andthe plurality of second elements are coplanar such that one of the firstelements has a first cross-section that overlaps a majority of a secondcross-section of one of the second elements.
 6. The method of claim 5,wherein each of the plurality of first elements has a width of betweenabout 10 microns and about 5000 microns and each of the plurality ofsecond elements has a width of between about 10 microns and about 5000microns.
 7. The method of claim 5, wherein the plurality ofsubstantially parallel first elements and the plurality of substantiallyparallel second elements are linear and intersect perpendicularly. 8.The method of claim 1, wherein the preformed pattern has a tapered crosssectional shape, being wider at the outer surface of the electricallyconductive mandrel.
 9. The method of claim 1, wherein the electricallyconductive mandrel has a first outer surface and a second outer surfaceopposite the first outer surface, wherein the first outer surfacecomprises a first preformed pattern and the second outer surfacecomprises a second preformed pattern.
 10. The method of claim 9, whereinthe first preformed pattern and the second preformed pattern aredifferent from each other.
 11. The method of claim 1, wherein theelectrically conductive mandrel is metallic, and wherein the at leastone preformed pattern comprises at least one pattern element as a cavityin the electrically conductive mandrel.
 12. The method of claim 11,wherein the pattern element is an intersecting trench having three metalsides.
 13. The method of claim 11, wherein the electrically conductivemandrel comprises a first layer on the outer surface, and wherein thefirst layer comprises at least one of a low-adhesion material or adielectric material.
 14. The method of claim 13, wherein the first layerextends at least partially into the pattern element.
 15. A method offorming an electrical component for a photovoltaic cell, the methodcomprising the steps of: i) electroforming a metallic article on anelectrically conductive mandrel, wherein the electrically conductivemandrel has an outer surface comprising at least one preformed pattern,the preformed pattern comprising a continuous grid pattern coupled to acell interconnection pattern, wherein the metallic article comprises aplurality of electroformed elements formed in the preformed pattern, theplurality of electroformed elements comprising a cell interconnectionelement coupled to a continuous grid having a plurality of firstelements intersecting a plurality of second elements, and wherein thecell interconnection element is configured to directly couple the gridto a neighboring cell; and ii) separating the metallic article from theelectrically conductive mandrel, wherein the plurality of electroformedelements are interconnected such that the metallic article forms aunitary, free-standing piece when separated from the electricallyconductive mandrel; wherein the preformed pattern has a height and awidth, wherein the ratio of the height to the width is an aspect ratio,and wherein the preformed pattern has an aspect ratio greater than 0.1;and wherein the plurality of electroformed elements are configured toserve as an electrical conduit for a light-incident surface of aphotovoltaic cell.
 16. The method of claim 15, wherein the step ofelectroforming comprises overplating the electroformed elements suchthat the electroformed elements extend above the outer surface of theelectrically conductive mandrel.
 17. The method of claim 16, wherein theoverplating forms a rounded top.
 18. The method of claim 16, wherein atop surface of the overplating provides an optical surface that aids inlight collection for the photovoltaic cell.
 19. The method of claim 15,wherein the preformed pattern has a tapered cross sectional shape, beingwider at the outer surface of the electrically conductive mandrel. 20.The method of claim 19, wherein the step of electroforming comprisesoverplating the electroformed elements such that the electroformedelements extend above the outer surface of the electrically conductivemandrel.
 21. The method of claim 15, wherein the cell interconnectionelement comprises a bus bar.
 22. The method of claim 21, wherein thecell interconnection element has a height that is less than the heightof the continuous grid.
 23. The method of claim 15, wherein the aspectratio is between about 0.1 and about 1.